Maybe I am reading too much into things but:
Days of Future Past led into Days of Future Present
Days of Future Present was a massive cross over with X-Men, X-Factor, New Mutants and Fantastic Four...all are owned by Fox

as for cast I don't think anyone is a lock or not a lock to be in this:

we saw a X-men movie where Storm, Wolverine and Rogue were X-men before Angel and Iceman
we saw an Avengers team founded by Cap, Black Widow and Hawkeye (yes Cap is considered a founder but the original team was IM, Thor, Hulk, Antman/giantman and wasp)